test_backend: Fix sentry DSN format
Latest sentry-sdk release now raises a BadDsn exception when DSN format does not match the expected one.
See https://jenkins.softwareheritage.org/job/DENV/job/run-all-tests/666/artifact/swh-vault.log
Latest sentry-sdk release now raises a BadDsn exception when DSN format does not match the expected one.
See https://jenkins.softwareheritage.org/job/DENV/job/run-all-tests/666/artifact/swh-vault.log